1. Field of the Invention
The present invention relates to a liquid-ejecting apparatus having a head with a plurality of liquid-ejecting units, each unit having a nozzle, and a liquid-ejecting method.
2. Description of the Related Art
As an example of a liquid-ejecting apparatus having a head with a plurality of liquid-ejecting units, each unit having a nozzle, an inkjet-type recording apparatus has been known. The inkjet-type recording apparatus such as an inkjet printer has been widely used in view of high-speed recording, inexpensive running cost, and easy colorizing, so that techniques for forming high-resolution and high-quality printed images have been developed.
For example, there is a serial-type print head in which while a print head is reciprocated in the full-width direction of a recording medium, ink is ejected from a liquid-ejecting unit arranged in the print head so as to form printed images. In the serial-type print head, a multipath system is employed. The multipath is a system in which when ink is ejected so as to form printed images during the reciprocation of the print head, for one line constituting printed images, ink is ejected from a plurality of liquid-ejecting units. Thereby, fluctuations in an ejecting direction and an ejection amount of ink ejected from each liquid-ejecting unit are able to be inconspicuous.
Also, in the inkjet printer, a pulse number modulation (a method for forming one pixel by a plurality of ink droplets so-called PNM) has been known. FIG. 20 is an explanatory view illustrating the pulse number modulation (PNM system). In this method, within one pixel region, ink droplets are continuously ejected plural times. It is not until the ink droplet landed at first is absorbed (permeated) into a photographic sheet that the next ink droplet is landed so that at least part of a region is overlapped with another region. FIG. 20 shows examples from an example where an ink droplet is landed once up to an example where ink droplets are landed five times. It is not until the ink droplet landed at first is absorbed (permeated) into a photographic sheet that the next ink droplet is landed, so that a plurality of ink droplets are united so as to form one large pixel. That is, the PNM is a system in which by adjusting the number of ink droplets ejected from each liquid-ejecting unit, the diameter of a pixel constituting a printed image is variably controlled so as to express gradation. In order to form high-quality printed images using such a PNM system, it is an important object to stabilize the ejection amount of an ink droplet ejected from each liquid-ejecting unit. As a technique relating to such an object, it is disclosed that during continuously ejecting ink, the amount of an ink droplet is stabled (Japanese Patent Publication No. 3157945 (page 3, FIGS. 5 and 8) for example).
The technique described in Japanese Patent Publication No. 3157945 relates to a technique in that a plurality of independent ink droplets for one pixel are defined as a ink droplet group and a pulse interval is set for a pulse signal for ejection from the same ejecting unit. Specifically, in a frequency band in which with increasing the pulse interval, the ejection amount per one droplet increases, the pulse interval is established so that the amount of each ink droplet of the ink droplet group is equalized with the amount of an ink droplet when a single ink droplet is ejected. Thereby, the pulse interval for equalizing the amount of each ink droplet of the ink droplet group ejected continuously is selected from a graph between a drive frequency and ink ejection amount characteristics, and the amount of each ink droplet can be constant using the selected pulse interval. However, this pulse interval is uniquely determined, so that it has not been arbitrarily established.
Incidentally, in response to the serial-type print head, there is a line-type print head having a number of head chips arranged corresponding to the entire width of a recording medium. If the line-type print head is applied to the technique described in Japanese Patent Publication No. 3157945, along with increase in the number of liquid-ejecting units, the electric power applied to a heating element provided in each liquid-ejecting unit may concentrate. In this case, the voltage of a power supply for supplying electric power to each heating element fluctuates, and as a result, high-quality images may not be formed (a first problem).
Also, in the technique described in Japanese Patent Publication No. 3157945, even if the pulse interval for equalizing the amount of each ink droplet of the ink droplet group ejected from each liquid-ejecting unit is selected from the graph about the ink ejection amount characteristics, by the effect of fluctuations of each component in the manufacturing process of the print head or changes in temperature in use, the amount of each ink droplet is liable to change, So that it has been difficult to stabilize the amount of each ink droplet of the ink droplet group ejected from each liquid-ejecting unit (a second problem).
Since in the line-type print head, a recording medium is moved relatively to the print head only in a direction perpendicular to the longitudinal direction of the print head so as to form printed images, the multipath system cannot be applied thereto. Therefore, fluctuations of each liquid-ejecting unit in the ejecting direction get lined-up along the imaging direction. If a head with fluctuations in the ejecting direction is used, although the printing must be actually performed as shown in FIG. 19B, there has been a problem of printed images with streaks and unevenness as shown in FIG. 19A (a third problem).
On the other hand, while the third problem being solved, in a liquid-ejecting apparatus having a head (line head) with a plurality of liquid-ejecting units arranged thereon, a technique enabling the PNM system, in which while liquid ejecting direction is controlled (deflected), one pixel is formed by landing ink droplets on one pixel region using a plurality of liquid-ejecting units, to be employed thereinto is proposed in Japanese Patent Application 2002-161928, which is assigned to the same assignee as this application.
However, in forming one pixel by landing ink droplets using a plurality of liquid-ejecting units, since a plurality of the liquid-ejecting units correspond to the one pixel, signal processing for ejection execution is complicated.
Furthermore, in forming one pixel by a plurality of ink droplets ejected from a plurality of liquid-ejecting units, as shown in FIG. 21, the displacement in landing positions of the ink droplets ejected from each liquid-ejecting unit tends to increase. Therefore, as shown in FIG. 21, when dots formed by a plurality of the ink droplets are united so as to form one pixel, the shape of the pixel is not approximated to a circle, and this may result in image-quality deterioration (a fourth problem).